As a conventional technology, for example, it is proposed in Japanese Patent Publication No. 2871891 that a ballast stores or estimates a lamp voltage when a lamp is lighted at rating, and in response to a value of the stored or estimated lamp voltage, controls lamp power at the time when the lamp is started next time.
However, in the technology described in Japanese Patent Publication No. 2871891, a principal object thereof is to control rising of a luminous flux of the lamp against a change thereof with time and to control power for starting of the lamp in a restarting state (hot restart). Hence, the technology described in Japanese Patent Publication No. 2871891 is not to control lamp power in order to absorb variations of startability of the lamp owing to an influence of impure gas in an inside of an arc tube, which have come to be known in recent years.
In recent years, from viewpoints of energy saving and the like, an extremely high efficiency lamp in which a light emission efficiency with respect to the power supplied to the lamp is improved has been ready to be introduced into the market. However, there is also a lamp in which a composition of gas filled in the inside of the arc tube of the lamp is changed in the course of pursuing such a high efficiency, whereby an amount of mercury is consequently reduced to an extreme extent. Such a lamp in which the amount of mercury is extremely small is prone to be affected by the impure gas (for example, hydrogen) in the inside of the arc tube in comparison with the conventional lamp. Specifically, in the lamp in which the amount of mercury is extremely small, an arc discharge generated immediately after the lamp is started becomes unstable. In particular, in the lamp in which the amount of mercury is extremely small, in a state where a temperature in the arc tube immediately after the lamp is started is low and the gas filled in the inside of the arc tube is not evaporated, movement of electrons between electrodes is not active. Accordingly, such a phenomenon that the arc discharge becomes unstable occurs significantly.
Hence, in a lamp into which a large amount of the impure gas is mixed, it is apprehended that the arc discharge immediately after the lamp is started may not become stable to thereby cause a failure of lighting.
As characteristics for relatively estimating the amount of impure gas contained in the inside of the lamp, there is mentioned a minimum lamp voltage Vmin after the lamp is started and shifts to the arc discharge. When the lamp power is supplied to the lamp by a ballast provided with certain lamp voltage-lamp power characteristics after the lamp is started, the larger the amount of impure gas in the inside of the arc tube is, the higher the minimum lamp voltage Vmin becomes, and the smaller the amount of impure gas in the inside of the arc tube is, the lower the minimum lamp voltage Vmin becomes. Specifically, as the lamp has a higher minimum lamp voltage Vmin, maintenance of the arc discharge becomes more unstable, and such a lighting failure of the lamp becomes more prone to occur.
Meanwhile, in order to stably maintain the arc discharge immediately after the lamp is started, more power just needs to be supplied from the ballast to the lamp. The maximum amount of impure gas mixed in the manufacturing process of the lamp, that is, the maximum value of the minimum lamp voltage Vmin is grasped, and even in a lamp in which a value of the minimum lamp voltage Vmin is the maximum, lamp power sufficient for maintaining the arc discharge is supplied thereto from the ballast immediately after starting the lamp. In accordance with this technique, it is possible to stably shift the lamp to rating lighting without causing the lighting failure thereof immediately after the lamp is started.
However, in the case where the above-described technique is adopted, excessive lamp power will be supplied to a lamp in which the mixed amount of impure gas is small, and there is a problem that a lifetime of the lamp will be shortened by an occurrence of abrasion of the electrodes. Moreover, even in the lamp into which the large amount of impure gas is mixed immediately after production thereof, when the rating lighting is continued for a long time therein, the impure gas is adsorbed to a getter provided in an inside of an outer tube, and startability of the lamp is gradually improved. Accordingly, the power after the lamp is started will be supplied excessively thereto in a similar way to the above-described lamp in which the amount of impure gas is small.
The present invention has been made in order to solve the problems as described above. It is an object of the present invention to provide a discharge lamp lighting device that stably shifts a lamp to the rating lighting without causing the lighting failure thereof even if the lamp is a lamp in which the amount of contained impure gas is large and the discharge after being started becomes unstable, and does not supply the excessive power to the lamp in which the amount of contained impure gas is small and the lamp power immediately after being started is not required much.